...
N7 rating under 120 means they DEFINITELY have not played each class fully. This is important because if they've spent time getting to understand each class's mechanics, they can better understand their role in the team and thus play better.
...
That's a crock of horse manure. (Your implication that lower ranked players suck... which is exactly what you implied - 'played each class fully' is a candy coated version of it - is what I'm referring to)
My Geth Grenadier (infiltrator) is level 20, has falcon X (the only acceptable choice for that build IMO), usually places 2nd in gold on any map I know in random PUGs, and has an N7 rating of 111. The last 5 or 6 matches on FBW practicing for the weekend challenge all made it with full extractions in around 20 mins... Didn't see any N7 rating increase... still at 111.... and in the random PUGs my damage/crowd control wil often carry a crappy team (as long as they stick near me and don't go running around the map like a bunch of crack-addled monkies).
This character has 2 roles. First, AoE crowd control that softens everything up nicely, second is damage and kills, and both are accomplished at the same time. This will net a fair share of kills but, more importantly, if you position yourself to stun and soften incoming waves at a choke pont, you make EVERYTHING easy pickings for your snipers and other squad mates (but snipers especially).
The fact that I refuse to promote and, instead, keep playing, has NOTHING to do with anything.